The present invention relates to a device for evaluating the curvature or shape of the cornea of the eye, and more particularly, to a corneal measurement device that assists with pre-operative or post-operative measurements of the cornea, with contact lens fitting and with the diagnosis of diseases of the cornea. Additionally, the present invention relates to an ophthalmic instrument for visualizing disruptions in the eye""s tear film which covers the cornea.
The cornea, being the front surface of the eye, provides its major refracting surface and is important to quality vision. Recently, a number of corneal surgical techniques have been developed for correcting visual deficiencies, such as near-sightedness, far-sightedness and astigmatism. In order to assist with such surgical techniques, a number of devices have been proposed or developed to evaluate the topography, i.e., the shape or curvature, of the cornea. In addition, such corneal topography techniques are useful for fitting contact lenses and for the diagnosis and management of corneal pathologic conditions, such as keratoconus and other ectasias. For example, prior to performing a corneal surgical technique to correct a refractive error, the patient is preferably screened using a corneal topography device to rule out the possibility of subclinical keratoconus.
Corneal topography is typically measured using a series of concentric lighted rings, known as a keratoscope pattern 5, shown in FIG. 1. In one typical embodiment, shown in FIG. 2, keratoscope pattern 5 is created by a keratoscope target 10, consisting of illuminated concentric rings which emit light rays which are projected onto the cornea of a patient""s eye 15. Light rays 12, 20 are reflected off patient""s cornea 15, and a portion of light ray 20 is captured by an objective lens 25 and focused onto an imaging system 30, such as a video camera. A computer 35 is utilized to compare the image captured on imaging system 30 with a stored reference pattern, or other known information, to identify any distortions in the captured image and thus calculate any deformations in the patient""s cornea.
While conventional corneal topography devices have achieved significant success, such devices suffer from a number of limitations, which, if overcome, could significantly enhance their accuracy and utility. In particular, earlier designs for topography devices have incorporated large keratoscope targets, causing the overall size of the prior art devices to be quite large. In an operating room or a doctor""s office, however, where space is at a premium, it is desirable to minimize the overall size of the topography device.
In addition, commercially available topography devices, such as the design illustrated in FIG. 2, typically measure the topography of only a relatively small area of the cornea. For example, in the design shown in FIG. 2, the light beam is emitted from a large, flat, backlit keratoscope target 10 and is then reflected off cornea 15. Thereafter, a portion of light 20 reflected off cornea 15 is focused by small objective lens 25 at the center of keratoscope target 10 onto imaging system 30, such as a CCD chip. Additional light rays 12 reflected from the peripheral portions of cornea 15, however, are not captured by objective lens 25 and are therefore not imaged onto imaging system 30. Therefore, such prior art devices are unable to measure the peripheral cornea.
To overcome this problem, prior art devices have attempted to capture the light rays reflected from the peripheral portions of cornea 15 by designing a keratoscope target 10xe2x80x2 in the shape of a cylinder or cone, as shown in FIG. 3, encompassing the peripheral cornea. In this manner, light rays emitted by cylindrical or conical keratoscope target 10xe2x80x2 will form a pattern 5 of illuminated rings which will be reflected off cornea 15. The reflected light rays, including light rays reflected off the peripheral portions of cornea 15, will be captured by objective lens 25 and imaged onto imaging system 30. To be effective, however, cylindrical or conical keratoscope target 10xe2x80x2 must be positioned very close to the eye, and thereby tends to impinge on the patient""s brow and nose. In addition to being potentially uncomfortable and potentially contributing to the spread of disease, the close approach of keratoscope target 10xe2x80x2 makes the design very error-prone, as a slight error in alignment or focusing causes a large percentage change in the position of the keratoscope rings relative to the eye.
In addition, current systems tend to provide poor pupil detection and do not accurately measure non-rotationally symmetric corneas, such as those with astigmatism. The location of the pupil is particularly important in planning surgical procedures for correcting visual deficiencies. In current systems, pupils are typically detected by deciphering the border of the pupil from the image of the keratoscope rings. This is particularly difficult with conventional designs, however, as the intensity transition from the black pupil to a dark iris is minimal compared to the intensity transition from a bright keratoscope ring image to a dark interring spacing. As a result, the pupil detection algorithms in current systems often fail.
Furthermore, current systems have difficulty detecting the edges of the keratoscope rings and difficulty separating ring images from background iris detail. Conventional corneal topography systems image the iris along with the keratoscope rings, as know as xe2x80x9cmiresxe2x80x9d. Particularly in patients having light-colored irises, however, the bright reflection from iris detail obscures the rings, thereby making detection of ring edges difficult. Finally, conventional devices utilize high intensity visible light to illuminate the keratoscope target and therefore cause discomfort to the patient. The high intensity light is required because relatively little light is actually reflected from the cornea and captured by the measuring devices.
As is apparent from the above discussion, a need exists for a more compact corneal topography device. Another need exists for a topography system that allows a large area of corneal coverage without the focusing problems and invasive approach of previous designs. A further need exists for a system incorporating improved pupil detection by using an image that does not include the keratoscope rings. Yet another need exists for a topography device providing improved separation of the corneal reflection of the keratoscope pattern from the iris detail. A further need exists for a topography system utilizing light levels that are not unpleasant for the subject undergoing measurement. An additional need exists for a topography device that permits accurate measurement of non-rotationally symmetric corneas, such as those with astigmatism.
Generally, according to aspects of the present invention, a method and apparatus for measuring the topography of the cornea are provided. The method and apparatus utilize a virtual image of a keratoscope pattern or other diagnostic pattern, which is projected at a desired distance in front of the patient""s eye. Since the topography is evaluated with a virtual image, there is no nose or brow shadow, allowing better coverage of the cornea and providing a design which is relatively insensitive to focusing errors.
In certain embodiments, however, it has been found preferable to position the image of the keratoscope pattern at some other location. For example, a virtual object of the keratoscope pattern may be formed just behind the cornea such that after being reflected from the surface of the cornea is re-imaged just in front thereof. Likewise, however, distortions in the cornea are observed in the reflected real image of the keratoscope pattern.
The disclosed topography system includes a structured light source, preferably consisting of an illumination source and a beam modulating system, to create the keratoscope pattern or other desired diagnostic pattern. In order to minimize discomfort to the patient, light emitted by the illumination source is preferably not in the visible range. In addition, the illumination source is preferably monochromatic.
In accordance with an aspect of the present invention, the beam modulating system may be embodied as a photographic slide film consisting of opaque markings on a transparent background, or a variable light pattern generator, such as an array of liquid crystal pixels, or an array of light emitting diodes. In this manner, the beam modulating system can provide flexibility in selecting pattern images to achieve various diagnostic abilities.
An optical assembly focuses the created pattern upon the cornea, and thereafter captures the image reflected off the patient""s eye and directs the reflected image toward an imaging system, such as a CCD, for processing. According to a feature of the invention, the optical assembly preferably includes means for preventing scattered light reflected from the patient""s iris from being imaged by the CCD. In one embodiment, a pair of polarizing filters having the same polarization attenuate the scattered light reflected from the patient""s iris, thereby permitting a clean image of the keratoscope pattern, as reflected off the patient""s cornea, on the CCD camera.
According to a further feature of the invention, the optical system achieves wide angle capture by including an aperture stop which is preferably conjugate with a point behind the corneal surface approximating the center of curvature of a normal cornea. Thus, reflected rays reaching the imaging system appear as if they originated at the center of curvature of the cornea.
Furthermore, the optical system may also be used for observing disruptions or abnormalities in the eye""s tear film by observing localized changes in the intensity of the acquired image of the cornea.
The corneal topography device preferably includes a centration illumination source and a focusing laser which are utilized to center and focus the corneal topography system relative to the patient""s cornea. During the centration and focusing operation, the structured light source used to generate the diagnostic pattern is preferably not illuminated. Thus, pupil detection is facilitated, since the pupil boundaries are not obscured by one or more rings of the keratoscope pattern.
In accordance with a further aspect of the invention, a method of calibrating the corneal topography device is disclosed. The method comprises the steps of: positioning a calibration sphere approximating the size of a cornea at a desired focal point; illuminating the calibration sphere with a diagnostic pattern; creating a first image on an imaging system of a reflection of the diagnostic pattern off the sphere; positioning a cursor on the imaging system at approximately the center of the first image; creating a second image on the imaging system of a reflection of a centration illumination source off the sphere; adjusting the position of the centration illumination source so that the second image is approximately centered around the previously positioned cursor; and storing the first image as a reference image for calculating topographical information about an unknown cornea.
Yet another aspect of the invention allows the disclosed topography system to be reconfigured as a perimeter to evaluate a patient""s field of vision. When configured as a perimeter, the structured light source is preferably embodied as a backlit liquid crystal array, a cathode ray tube or an array of light emitting diodes. To measure the patient""s visual field, the patient observes a virtual image of the pattern produced by the structured light source, which is projected at a distance in front of the patient""s eye. In addition, an infrared laser illuminates the patient""s pupil with an infrared beam. A reflection of the infrared beam scattered off the patient""s iris is imaged on the CCD.
According to a further feature of the invention, the optical system includes means for attenuating light which is reflected off the patient""s cornea during the visual field measurement, such as a pair of perpendicular polarizers positioned in the optical path. The system monitors fixation by tracking the movement of the pupil, using the scattered infrared image. When the center of the pupil moves beyond a predefined threshold, an alarm indicates when fixation is lost.